In the past, a coating type of magnetic recording media has been widely employed. This type of media is generally obtained by coating on a non-magnetic support a magnetic coating composition prepared by dispersing a powdery magnetic material into an organic binder and drying it.
However, a coating type of a magnetic recording media has the disadvantages that saturation magnetization is low due to ferromagnetic particles being mainly used as the metal oxide particles and the density of ferromagnetic particles in the magnetic layer cannot be increased due to organic binders being included in the magnetic layer. Therefore, this type of media is not suitable for high density recording and the manufacturing steps are complicated.
In recent years, there has been a demand for high density recording and a magnetic recording medium comprising a non-magnetic support having provided thereon a thin metal film has been developed to respond to the above demand. Such a recording medium is prepared by forming on a mon-magnetic support a thin metal film by a vapor deposition method such as vacuum evaporation, sputtering or ion-plating or a metal plating method such as electroplating or non-electrolysis plating. The magnetic composition is not limited to metals, but metal is the typical magnetic component.
The above described medium is hereunder referred to as a thin metal film type magnetic recording medium. As the above described medium does not use organic binders, it is also referred to as a non-binder type magnetic recording medium.
In a thin metal film type magnetic recording medium, the thin film is composed of ferromagnetic metal having high saturation magnetization without containing binders. Therefore, such a medium can have higher coercive force and a thinner film than a coating type medium, is scarcely affected by demagnetization in a short wavelength region, can attain high density recording and can be prepared by a simple manufacturing process. Therefore, this type of media has recently drawn much attention.
However, even though the thin metal film of the thin metal film type magnetic recording medium appears to have a uniform and smooth metal surface, microscopically it has a structure composed of coarse metal particles and readily corrodes.
For the above reason, the thin metal film type magnetic recording medium has poorer weather resistance and corrosion resistance than the coating type magnetic recording medium. Particularly, with regard to a magnetic recording medium such as a cassette tape or a video tape, the surface thereof is rubbed by a magnetic head during the recording and reproducing operation. If even a slight amount of corrosion is present on a thin metal film surface, it is removed by the rubbing action, thereby causing head clogging and scratching of the magnetic head and the magnetic recording medium.
Further, the thin metal film type magnetic recording medium has poor durability. As the thin metal film is smooth, friction is great and stick-slip takes place. Still durability (durability at still mode) on a VTR is poorer than that of the coating type magnetic recording medium.
Many attempts have been made to improve weather resistance and durability, such as the method of providing a surface nitrated treatment by ion plating (Japanese Patent Application (OPI) No. 33806,75), the method of providing a silicon nitride film by sputtering (Japanese Patent Application (OPI) No. 30304/78), the method of forming a non-magnetic surface layer by exposing a magnetic layer to discharge in a nitrogen gas atmosphere (Japanese Patent Application (OPI) No. 85403/78) and the method of providing a nitrated metal thin film on a thin magnetic metal film (Japanese Patent Application (OPI) No. 143111/79). (The term "OPI" used herein means an unexamined published application.)
Providing a magnetic thin film composed of iron nitride or of iron and iron nitride is suggested for the non-binder type magnetic recording medium as disclosed in European Pat. No. 8328 and in Japanese Patent Application (OPI) No. 87809/84.
One of the inventors of the present invention previously suggested in Japanese Patent Application (OPI) No. 54023/86, of which he is one of the co-inventors, a magnetic recording medium comprising a non-magnetic support having provided thereon a thin magnetic film mainly composed of iron nitride oxide having the following formula. EQU FE.sub.1-X-Y N.sub.X O.sub.Y (0.25.ltoreq.X+Y&lt;60)
However, the above earlier invention has the disadvantage that the film thickness must be increased to effectively improve weather resistance and durability and the drawback that even though the magnetic recording medium mainly containing iron nitride or iron nitride oxide can improve weather resistance, magnetic characteristics are decreased.